The best way to verify the bloodline of your yellow Labrador is to check with the American Kennel Club by calling their customer service line or using their online pedigree tools at AKC.org. This organization tracks purebred dogs and verifies pedigrees.
If you are buying your puppy from a breeder, the breeder should have registered the litter with the AKC before you take it home. Registration must be done shortly after birth, and the dog cannot be registered with the AKC if the breeder does not do it. If your breeder did not register the litter, you cannot verify the dog's pedigree.
Disreputable breeders have occasionally lied to buyers and the AKC by registering a mixed-breed litter. This may occur when a female dog is accidentally allowed to mate with a male of a different breed, but the breeder claims that a different dog is the sire. This scam is has become more difficult to accomplish due to DNA testing, but, as of 2015, not all litters are required to be tested. If you suspect something like this has occurred, you can contact the AKC with your concerns. The organization may investigate and discipline the breeder.
Choosing the right breeder can give you confidence that your puppy has the proper bloodlines and can help you avoid many health and behavioral problems as well.